1. Field of the Invention
This invention relates to display devices with touch screens.
2. Description of the Related Art
Touch screens are becoming increasingly common in consumer electronics applications where an LCD display is present in a device e.g. mobile phone, PDA or camera. User interaction via a touch screen saves the space required for key inputs and therefore allows a larger display area for a given size of device.
Of the possible physical effects used to locate the “touched” position on such a screen, sensing the capacitance change induced between orthogonal sets of electrodes, or between a grounded stylus and individual electrodes, promises the highest resolution while integrating most easily with existing manufacturing processes.
The conventional approach to add touch screen functionality is to simply laminate the necessary electronic structure on top of a display. A potentially cheaper solution that is being investigated is to integrate as much as possible of the touch screen structure into the display. This also has the advantage of adding value to the display itself. In either case, there is a need to maximise the signal so that an accurate “touched” position can be determined.
The signal can be maximised by varying the fill factor of the sensing electrodes (in other words the proportion of the overall area which is occupied by the electrodes) in some way. For example diamond shape pads can be added to a linear array of wires. There are a variety of proposals concerning the best fill shapes to use.
One unavoidable feature of touch screens on displays is that one set of the orthogonal electrodes will be close to an electrical ground plane within the display itself. This tends to reduce the sensitivity, as the ground plane “attracts” electric field lines that would otherwise extend out of the device, these field lines cannot therefore be influenced by touching the screen resulting in the reduced sensitivity.